Some data from ATLAS at the LHC

By Marcus Wilson 15/12/2010

No, the Large Hadron Collider hasn’t vanished. It might not be so prominent in the news as it was two years ago, but it is quietly colliding protons together and generating lots of useful data for analysis.

Here’s a couple of bits which I gleaned in Melbourne

1. What lies inside a quark (if anything?).  Us physicists are happy with the notion that at the centre of an atom lies a nucleus, consisting of protons and neutrons, and each proton and neutron contains three quarks. (For the case of a proton, it’s two ‘up’ quarks and one ‘down’ quark; for the neutron its two ‘down’ quarks and one ‘up’. Protons and neutrons are actually very, very similar things.)  But is the quark made of anything? How could we tell? Basically, the way you do this is to collide protons together (i.e. 3 quarks on 3 quarks) and carefully analyze the statistics of the scattering. At what angles are the protons scattered? Is there fine-structure in the scattering pattern? This is exactly what Rutherford did with Geiger and Marsden’s alpha-particles on gold-foil results to determine that there must be a nucleus to an atom. In the case of the gold-foil, the structure in the pattern is pretty obvious. In the proton-proton case, it’s not. In fact, results from the ATLAS experiment at the LHC fail to indicate any structure at current energies (3.4 TeV).  (In particle physics, higher energies equate to probing smaller distances). So we can conclude that IF there is structure (and it’s a big if), it must appear at energy scales larger than 3.4 TeV. So far, the quark remains ‘fundamental’.

2. Can we find dark matter?  Dark matter is what is thought to make up 23% of the mass/energy of the universe. It has the annoying property that you can’t see it – in fact it doesn’t interact with any electromagnetic things. So why do we think it’s there?  If you study the way galaxies are moving, knowing what we know about gravity, we come to the conclusion that there simply isn’t enough visible mass in a galaxy to account for its movement. Galaxies seem to be more massive than we can account for by ‘counting’ stars in them. This missing mass is called ‘dark matter’.  (N.B. There’s also dark energy, that makes up about 73% of the universe, which is another thing, but I won’t go there today.) So, what is it? We don’t know, but there are theories. Moreover, these theories are testable – in that you can use them to make predictions about what might be observed in the LHC. So people are busy analysing results of collisions to see if there are features observed in the LHC that can only be explained by the theories of dark matter. If there are, that’s strong evidence for the ‘discovery’ of dark matter.  I have to say that listening to a couple of talks I was impressed at the size of the research effort on theories of dark matter – given that this stuff hasn’t actually been observed yet. It must take a bit of faith to spend your PhD studying something that might not even exist.